1. Field of the Invention
This invention relates to electrophotographic photosensitive members and more particularly to an electrophotographic photosensitive member improved in the stability of performance characteristics by employing an acrylic resin having a specific glass transition point and acid value, as a binder resin for forming its photoconductive layer.
2. Description of the Prior Art
Electrophotographic photosensitive members adopt various constructions in order to obtain desirable characteristics or to meet varieties of electrophotographic processes applied. One of the representative constructions has a photoconductive layer, as an image retaining layer, on a substrate; another one is provided with a photoconductive layer, as an image retaining layer, on a substrate and with an insulating layer laminated on the photoconductive layer. These two constructions are widely adopted. Photosensitive members of the former construction are used for image formations according to the most popular electrophotographic process, i.e., the process comprising charge, image exposure, development, and if necessary, transfer. The insulating layer of the latter construction aims at protecting the photoconductive layer, improving the mechanical strength of photosensitive members, improving dark decay characteristics, or adapting photosensitive members for a specific electrophotographic process. Typical examples of photosensitive members having such an insulating layer or of electrophotographic processes employing these photosensitive members are described in U.S. Pat. No. 2,860,048, Japanese Patent Publication No. 16429 (1966), and U.S. Pat. Nos. 3,146,145, 3,607,258, 3,666,363, 3,734,609, 3,457,070, and 3,124,456.
An electrophographic process is applied to an electrophotographic photosensitive member to form electrostatic images, which are then developed to visualize.
Photosensitive members hitherto used have a tendency such that the dark area potential, light area potential, and intermediate area potential cannot have their respective constant values during repetition of charge and exposure or after the recess time, and the developed image densities (D max) are therefore labile. This is attributable to a light memory or charge memory produced in photosensitive members. These phenomena are largely affected by binder resins for use in bonding cadmium sulfide to form a barrier layer.
When a binder resin deficient in resistance or stability to light, humidity, or heat is used for bonding cadmium sulfide, the binder will adversely affect the stability of the resulting photoconductive layer forming material, thus often producing unstable photosensitive members, so that its use for the production of photoconductive layers is much restricted.
In particular, when a humidity-labile binder resin is used, repeated charges of the photosensitive member under high humidity conditions will cause the degradation of the photoconductive layer, thus raising the problem of durability or stability in a high humidity environment.
Meanwhile, the binder resin for this purpose is desired to have suitable properties as a binder, that is, a high ability to disperse cadmium sulfide, good coating workability so as to give a desired film thickness, strong adhesion to the substrate, and ability to give the photoconductive layer a good mechanical durability. A few sorts of binder resin have sufficiently satisfied electrophotographic performance characteristics of photoconductive layer and suitable properties as a binder, as a whole.
The environmental temperature is another factor causing variations in performance characteristics of photosensitive members. Many types of photosensitive member are liable to undergo an influence of temperature changes from low to high and in the reverse direction, thereby charge bearing characteristics thereof being deteriorated.